LATEST COKER DESIGNS INCREASE LIQUID YIELDS, REDUCE EMISSIONS

Modern coker designs incorporate features that maximize liquid yields, enhance safety, and reduce emissions. Careful engineering of delayed cokers can as much as eliminate liquid effluents.

Coking and other bottom-of-the-barrel processes were the topic of presentations at Foster Wheeler USA Corp.'s Heavy Oils Conference, in June at Orlando. At the conference, John D. Elliott, Foster Wheeler, Perryville, N.J., provided a perspective on coker design considerations.

The delayed coking process will play an increasingly important role in the modern refinery, said Elliott, because of its ability to convert heavy vacuum residues to distillates and petroleum coke.

The flexibility, inherent in delayed coking permits refiners to process a wide variety of crude oils, including those containing heavy, high-sulfur resids. These crudes frequently are bought at a discount, and if economically convertible to light distillates, can be a substantial factor in the refiner's cash flow development.

SPECIALTY COKE

Two types of coke are produced for specialty end uses-anode and needle grades. Anode-grade coke is used to produce carbon anodes consumed in aluminum refining. Needle coke is used to manufacture the graphite electrodes used by the steel industry in electric arc furnaces.

Elliott listed several features of anode-coker design, as compared to fuel-grade cokers:

More severe temperature and pressure conditions
Lower corrosion rates
Higher-energy jet pump and coke-cutting systems
Smaller-diameter drums.

And to minimize the impact on fractionator loads in anode-grade cokers, a portion of the feed can be directed over the shed wash section in the bottom of the column. This provides direct heat transfer to the feed and reduces the need for additional pumparound or overhead reflux.

Current design trends for needle cokers include:

Feedstock pretreatment using desulfurization, solvent extraction, or thermal treatment
Higher pressure and recycle ratio (typically 50-90 psig and 60-100% recycle)
Higher temperatures
Longer cycles
Thicker drum walls
Higher jet-pump discharge pressure
Smaller coker drums (needle coke has been cut only in drums with diameters of 24 ft and less)
Incorporation of post-treatment operation to extend mesophase reactions.

In addition, the coker heater must be mechanically capable of withstanding the more severe operating conditions.

FUEL-GRADE COKERS

The prominent design for new coker projects is the fuel-grade coker. This process produces coke as an unwanted byproduct of liquid yields. Fuel coke is generally high in sulfur ( 3.0 wt %) and metals (Ni + V 300 ppm). Shot coke can have a low HGI and, if graded "high-sulfur," can result in a low sales price.

Traditional markets for fuel coke are export sales and cement kilns, with some utility stations firing coke. Lower prices for high-sulfur coke are providing opportunities for coke-fired cogeneration units that produce steam and electricity for the refinery and for export (OGJ, April 5, p. 38).

Maximum liquid yields are obtained by designing for low-pressure and ultralow- recycle operation. Typically, operating pressures are less than 15 psig and recycle ratios, 5% or less.

Refiners generally operate cokers at the lowest recycle ratios permissible, in terms of unit operations and coker gas oil quality, said Elliott. Even older cokers benefit from decreasing the recycle ratio to reduce coke yield.

Table 1 shows the yield variation that results from reducing recycle ratio at constant drum pressure. Although ultralow-recycle operation reduces coke production and increases liquid yield, heavy coker gas oil (HCGO) quality is affected. As HCGO yield increases, the distillation endpoint increases, together with its carbon residue and metals content. And HCGO specific gravity decreases.

Foster Wheeler designs for low-pressure coking include:

Increased compressor size and horsepower
Minimally increased fractionator and vapor-line size
Use of low-pressure-drop components in the vapor path.

The compressor loads required for low-pressure operation can have less impact on costs than once thought, said Elliott. A recent study showed that frame size was unchanged over a pressure range of 15-30 psig.

For a medium-sized coker, the fractionator need only be about 1 ft larger in diameter for 15-psig coke-drum operation, as compared to a 25-psig. And specialty valves in the drum overhead line are ball or wedge plug valves with a 90% port opening for reduced pressure drop.

The fractionator overhead condenser is also designed for reduced pressure drop. The concern over coke deposits in the drum overhead causing high coking pressures is handled by injecting gas oil quench at rates that do not adversely, affect recycle ratio. New designs use a spray nozzle to achieve this.

Table 2 shows results of a study evaluating the economics of coking 33,000 b/sd of heavy, high-sulfur vacuum resid at constant ultralow recycle ratio and 15-30 psig. The incremental annual revenue generated by reducing the pressure from 25 psig (base case) to 15 psig outweighed the differential equipment and utility costs. Payout on this pressure reduction was projected to be less than 1 year, said Elliott, even considering lower product costs.

DESIGN TRENDS

Many of the new, large cokers are being sized for reduced recycle operation-often as low as 18 hr coking, said Elliott. The process design engineer can reduce coke-drum size and perhaps save the unit from requiring and extra pair of drums, as might be required with a conventional 24-hr cycle. This design, however, imposes burdens on decoking operations.

Another design trend has resulted from efforts to enhance the safety of the drum unheeding process. Foster Wheeler's new bottom unheading system is designed to remotely manipulate the bottom head and the telescoping chute. The first system has operated successfully for more than 1 year.

Four-way, ball switch valves also are used on most new delayed cokers. These valves are easier to operate and entail less maintenance.

Another safety concern results from nonideal coke quenching, says Elliott, which causes rapid vaporization of water during coke cutting. This can result in "eruptions" of steam and coke particles. These "blow-backs" can be mitigated by:

Maintaining adequate steam flow during the drum switch (This prevents closure of internal coke-bed passages, which could impede the flow of water quench.)
Slowing the quench rate
Ensuring complete quenching by installing a high-level detector on new drums
Cutting the flow of water to the drill stem when a warning rumble is heard.

Operators should watch for uneven drum cooling and be ready to cut the flow of cutting water quickly. Foster Wheeler suggests that the operator check the amount of drum movement at the top deck prior to cutting. This movement-known as the banana effect-is caused by uneven drum-wall cooling.

HEATERS

Foster Wheeler offers two types of coker heaters: the standard type and the newer terrace-wall, double-fired type. The standard type normally provides 1-year run lengths and typically is adaptable to on-line spalling should premature coking occur.

On-line spalling removes coke deposits in the furnace tubes without the conventional necessity, of shutting down the furnace for steam/air decoking. Spalling is performed on one tube at a time and does not significantly reduce unit throughput. The process can allow coker furnaces to achieve runs of 1 year or longer, even when the heater is fired more than 50% beyond its design.

The double-fired furnace features shorter in-tube residence times and fires the tubes from two sides to reduce the peak flux rate. This design is less prone to coking, but slightly more expensive, said Elliott. The first commercial unit has been in operation for more than 1 year.

Furnace metallurgy is also being enhanced. New designs made from enhanced 9 Chrome, which costs about the same as standard 9 Cr tubes, should permit skin-tube temperatures to be increased by 100 F.

FRACTIONATOR

Operation with recycles of 5% or less is standard for new fuel-grade cokers. There is a trend, however, to retrofit this design to older cokers. Design details include a fractionator spray-chamber with wash zone, improved control of overhead line quench, and minor tower modifications.

Shed baffles are placed well below the spray chamber, requiring that wash-zone heat and mass transfer take place without the benefit of packing internals, which can be subject to coking during an upset. This design, coupled with a controlled coke-drum overhead line quench on differential temperature, permits wash-oil turndown rates as high as 25% of design.

Other tower modifications involve segregating the colder, liquid feed pool in the bottom of the fractionator from the overhead vapors to reduce unwanted gas oil condensation, thereby holding recycle to a minimum.

A number of refiners operate "zero-recycle" cokers (Fig. 1). These units actually operate at true recycle ratios of 2-4% because of the condensation caused by minimum overhead-line quench oil, wash-zone reflux (wash oil), and heat losses. This type of operation, however, can produce heavy coker gas oil with much higher carbon residue and Ni + V content.

The design in Fig. 1 is unique in that the HCGO is mostly condensed by an in-line quench upstream of the fractionator, and the HCGO product is withdrawn from the fractionator bottom. This setup requires a vessel separate from the heater surge drum, but significantly reduces the size of the fractionator.

The effect of a coke-drum switch on fractionator operation can be minimized by:

Raising the fractionator draw-pan levels to 100%. This should be done slowly, spanning 1/2-1 hr, prior to switching coke drums.
Using a "slow-switch" technique (10 min) when switching drums.
Slowly reducing pumparound and product flows to 75-85% of normal prior to the drum switch.
Raising the outlet temperature of the coking coil to maintain the heat input to the fractionator as close as possible to normal.
Reducing or eliminating the coke-drum overhead line quench.

COKE-HANDLING SYSTEMS

The two coke-handling systems most frequently specified are the "pit" and "pad" types (Figs. 2 and 3). The pit system uses an overhead "clamshell" bucket on a bridge crane to remove cut coke. The pad system employs a front-end loader to remove the coke.

Typically, the pit is used on larger cokers and the pad on smaller ones (< 18,000 b/sd). Although both designs are open, they are environmentally acceptable in most locations and have been permitted recently, said Elliott. Totally enclosed systems also are available, for minimum environmental impact.

Fines removal from the sedimentation maze is traditionally performed by the bridge crane or front-end loader. Putting a front-end loader down into the maze however, is cumbersome at best. Foster Wheeler's new pad design has a primary settling basin for accumulating most of the fines (Fig. 3).

The settled fines are pumped out of the maze by specialty slurry/sludge pumps. The fines are recovered by pumping the coke slurry over sieve bends. This system also can be used to remove fines from the maze, which is sloped to facilitate this process.

Water from the maze discharges over a weir into a clear-water sump. This sump contains special vertical water pumps that return the water to the decoking-water storage tank. Carryover of fines can be rectified by installing an underflow weir at the discharge end of the maze.

The clear water may contain very low levels of entrained fines, which can be removed upstream of the jet pump. To achieve this, the decoking-water tank is sized as the final clarification device and hydraulic systems remove the settled coke from the tank.

ENVIRONMENTAL IMPACT

Also at the Heavy Oils Conference, Foster Wheeler's Michael J. McGrath outlined engineering strategies for delayed cokers in environmentally sensitive areas.

Although the U.S. Environmental Protection Agency (EPA) has not promulgated regulations specific to delayed cokers, several current regulations impact coker design:

Prevention of Significant Deterioration program and its required Best Available Control Technology analysis
Effluent Standards and Guidelines (40 CFR Part 419)
VOC Emission New Source Performance Standards (40 CFR Part 60, Sub-parts A, J, Kb, VV, GGG, and QQQ)
Nonattainment Area Review for facilities in regions that exceed ambient air quality standards.

The impact of these regulations on current coker design primarily focuses on the coker heater and minimization of fugitive particulate and volatile organic compound (VOC) emissions. Regulations expected to impact delayed coker design in the future are the Clean Air Act Amendments, the anticipated reauthorization of the Clean Water Act, and EPA's decision to list additional refinery waste streams (OGJ, Dec. 16, 1991, p. 39).

Environmentally driven improvements in delayed coking have been directed at:

Eliminating fugitive coke dust emissions
Recovering hydrocarbon vapors
Reducing coker heater emissions
Reusing wastewater within the coker
Reducing miscellaneous emissions while providing for disposition of refinery wastes and slops.

FUGITIVE COKE DUST

One approach to eliminating coke dust emissions is, as Elliott mentioned, a totally enclosed coke-handling system. Two types of enclosed systems are operating commercially-gravity flow and slurry-type systems.

In the gravity flow-type system, the coke is cut from the drum, crushed, and fed by gravity into a specially designed dewatering bin, where the water is separated from the coke (Fig. 4). The dewater coke is fed to an enclosed conveyor system with dust collection points.

In the slurry system, the coke is cut from the drum, crushed, and fed via a pipe/sluiceway to a slurry pump (Fig. 5). The slurry pump then pumps the coke-water slurry to the dewatering bin.

One advantage of the slurry system, said McGrath, is that the coke can be transported a considerable distance by pipeline, thus eliminating dust emissions.

The bottom of the coke drum in these systems is enclosed by means of a telescoping chute. There are several options for mitigating emissions from the top of the drum, including water sprays, specially designed top covers, and a scrubber with an induced-fan ventilation system.

Recently permitted delayed cokers have utilized such dust-suppression tactics as including truck wash stations, limiting truck traffic to paved roads, and covering truck coke-loads securely.

VAPOR RECOVERY

In older cokers, hydrocarbon vapors left in the drum at the end of the cycle were recovered by steaming the filled coke drum to the fractionator. Any remaining hydrocarbon vapors were vented to atmosphere through the blow-down system.

As environmental regulations became more restrictive, vapors from the blowdown system were vented to the flare. But, said McGrath, designing fuel-grade cokers for lower operating pressures has provided an opportunity to recover blow-down vapor using the wet-gas compressor, with only a small residual being flared.

Fig. 6 shows a modern, enclosed coker-blowdown system. The use of gas-recovery systems enables the refiner to recover all of the blowdown vapor. These systems can be combined with flare-gas recovery, said McGrath, or used "stand-alone."

HEATER EMISSIONS

Coker heater emissions have been reduced through the use of high-efficiency furnaces that utilize air preheat. The use of low-sulfur fuels-preferably natural gas-has significantly reduced sulfur emissions.

A significant portion of the absorbed duty of a fuel-grade coker heater can be recovered either as steam feed preheat or as reboiling media in the coker gas plant.

NOx emissions can be controlled by using low-NOx burners or through catalytic NOx removal. Low-NOx burners can meet South Coast Air Quality Management District (Los Angeles basin) Rule 1109 (0.03 lb/MMBTU) and Rule 1146 (40 ppm[vol]).

WASTEWATER REUSE

The delayed coker is a net consumer of water. Water is lost from the unit as moisture in the coke, as evaporation, and as sour water to the sour water stripper. If there is a calciner associated with the coker, it is also a consumer of water.

Wastewater produced by the coker may be reused directly by diverting a portion of the water condensed in the blowdown system to the coke-cutting water. Coker wastewater can be reused indirectly by returning stripped sour water as cutting water makeup.

The key to reusing coker-blowdown water, says McGrath, is in selecting the appropriate time in the cycle to recover the water. The water condensed during drum steamout is typically high in H2S. As the cycle proceeds from steamout to quench, the rate of H2S evolution slows.

Depending on the sulfur content of the feed and its distribution in the products, the water quality will become acceptable for diversion from the sour water stripper to the cutting water system. According to McGrath, in most cases stripped sour water from the coker can be reused as makeup water.

A totally enclosed coke-handling system will reduce the odors that can result from reusing water associated with refinery sludges. Because of its high temperature and long residence times, a calciner also can dispose of certain "difficult" wastewaters, including high-phenolic ones.

A modern coker, concludes McGrath, can be considered a zero-liquid-effluent unit.

OTHER EMISSIONS

Refiners have reduced fugitive VOC emissions by implementing leak detection and repair programs, as required by 40 CFR Parts VV and GGG. These regulations also have reduced VOC emissions by eliminating open-ended valves or lines, enclosing sample connection systems, and necessitating barrier fluids on compressor seals.

VOC emissions can be further reduced, during the design phase, by minimizing components that may leak (valves or flanges), or by using leakless technologies. Double-seal pumps or sealless pumps can be specified.

WASTE-OIL DISPOSAL

New U.S. regulations have caused refiners to look to the coker as a place to reclaim oily wastes. Depending on the oil and solids content of refinery wastes, several disposal options are available.

Fig. 7 shows one scheme for disposing of high-water-content wastes containing some solids. Waste heat from the blowdown operation-supplemented as required by low-level heat from the fractionator-is used to dewater the sludge in the blowdown drum. Water and light oils are recovered in the blowdown settling drum.

Heavy oils and any contained solids are fed to the coke drum via the coker heater. The heavy oil is vaporized and recovered in the coker fractionator.

Other options include integrating a carrier-oil sludge drying system, such as Carver-Greenfield (Fig. 8). Also, said McGrath, a limited amount of sludge can be injected during the quench operation.

The disposal of waste tube oil and scrap plastics in cokers has also been proposed.

In terms of product quality, injecting sludges-especially those containing solids-probably will deteriorate the quality needle coke. Anode-grade coke, however, should not be severely impacted by most refinery wastes, said McGrath, if added in modest amounts. He added that a good understanding of which contaminants may be present in the sludge is required because anode coke has significant commercial value.

As long as "problem" contaminants such as lead are not present in the waste streams, processing wastes in a fuel-grade coker should have no impact on the marketability of the coke.

Several improvements in the design of delayed cokers have improved the cleanliness and efficiency of the process. And the ability of a coker, concluded McGrath to reuse water and upgrade refinery wastes is a plus in this era of minimal emissions.